Methods and apparatus for enhanced sputter-ion pump operation



Dec. 1, 1964 s. L. RUTHERFORD METHODS AND APPARATUS FOR ENHANCEDSPUTTER-ION PUMP OPERATION Filed Aug. 14, 1961 United States Patent OMETHDS ANB AP?ARATUS EUR ENHNCEB SPUITERN PUMP lERATlN Sherman L.Rutherford, Palo Alto, Calif., assigner to Varian Associates, Palo Alto,Calif., a corporation of California Filed Aug. 14, wel, Ser. No. 131,225

13 Claims. (Cl. 23t3--59) This invention relates to methods andapparatus for enhancing the pumping of sputter-ion pumps. Moreparticularly, this invention relates to enhanced hydrogen and hydrogenisotope pumping with sputter-ion pumps.

The pumping mechanism for such chemically active gases as oxygen landnitrogen in cold-cathode gas discharge sputter-ion pumps of the typedescribed and claimed in U.S. Patent No. 2,993,638, issued to L D. Hallet al. on July 25, 1961, is dominated by chemical combination withsputtered active metal atoms from the cathodes. These gases causecathode material such as, for example, titanium or zirconium to besputtered rapidly enough that approximately one atom of gas is pumpedfor each ion formed in the discharge. Most of the pumped oxygen andnitrogen reside on the anode of the pump in chemical combination withsputtered cathode material.

In contrast, hydrogen and its isotopes are so light that their ionsproduce comparatively little sputtering. Despite this, it is found thatpresent sputter-ion pumps employing cathodes made of, for example,titanium or zirconium generally display ya hydrogen pumping speed whichis more than twice that for oxygen or nitrogen. It has also beenestablished experimentally that most of the hydrogen is pumped bydiifusion into the cathodes rather than by chemical combination at theanode. In addition, signiiicant residual pumping (continued pumpingafter the discharge voltage is removed) occurs, indicating that hydrogencan be pumped by direct molecular contact with the cathode surfaceswithout the necessity for dissociation, ionization or ion burial.

These observations suggest that two factors of dominant importance inthe pumping of hydrogen are (l) cathode surface conditions which affectthe rate at which incident molecular hydrogen is adsorbed, and (2)solubility of the bulk cathode material for hydrogen. Surface conditionsare strongly affected by oxide films and layers of adsorbed gas, whilesolubility is influenced by temperature, pressure and impurities ordefects in the cathode material.

The present invention deals with the first of the above factors in thatit provides a method of sputter-ion pump treatment which results in adramatic enhancement of the pumps hydrogen pumping speed. The pumping ofa gas which is chemically inert wi-th respest to the cathode materialand whose molecules have a greater mass than those of hydrogen producesa surface cleaning effect on the cathodes of typical sputter-ion pumps.This cleaning effect greatly increases the cathodes hydrogen sorptionrate and thus provides a pump with greatly increased 'hydrogen pumpingspeed (typically 10 times the normal hydrogen pumping speed).

This invention also deals with the second of the above factors in thatit provides a method and apparatus for further increasing the hydrogenpumping throughput a-t high pressures (above lrmm. Hg). Ordinarily,sputter-ion pump operation at high pressures of hydrogen is diicultbecause the pump current is proportional to pressure and the increasedpower which must be dissipated causes cathode heating with subsequentthermal re-evolution of the previously absorbed hydrogen. The novelmethod and apparatus of this invention greatly reduces the high pressurepower inputto the pump to thereby reduce hydrogenre-evolution whilestill retaining a steady and relatively high value of pumping speed.This high pumping speed is possible because of the molecular hydrogenpumping characteristic of sputter-ion pumps.

The object of the present invention therefore is to enhance the pumpingof hydrogen and hydrogen isotopes with sputter-ion pumps.

One feature of the present invention is the provision of a method andmeans for `treating sputter-ion pump cathodes by pumping a gas which ischemically inert with respect to the cathode material and whosemolecular mass is greater than that of hydrogen to provide a pumpexhibiting a greatly increased hydrogen pumping speed.

Another feature of the present invention is the provision of a method`and means of the above featured type wherein the treatment of thesputter-ion pump cathodes is made with argon.

Another feature of the present invention is the method and means formaintaining a constant average pump input power during high pressureoperation to prevent excessive cathode temperatures and associatedhydrogen reevolution.

Still another feature of the present invention is the provision of amethod and means for producing an intermittent energization of 4asputter-ion pump thereby reducing the pumps input power while retaininga steady and relatively high value of hydrogen pumping speed.

These and other features and advantages of the present invention willbecome more apparent upon a perusal of the following specification takenin connection with the accompanying drawings wherein,

FIG. l is a diagram showing the variation of hydrogen pumping speed withtime due to hydrogen pumping and argon sputtering,

FIG. 2 is a diagram showing the residual hydrogen pumping characteristicof a sputter-ion pump,

FIG. 3 is -a diagram indicating the relative portions of hydrogenpumping speed which are due to ion pumping and molecular pumping for anargon treated sputter-ion Pump,

FIG. 4 is a diagram showing the dependence of teinperature of thehydrogen solubility rate for a sputter-ion Pump,

FIG. 5 is a diagram showing power versus pressure curves for aYtypically operated sputter-ion pump and one operated in accordance withone embodiment of the present invention, and

FIG. 6 is the circuit diagram of a power supply which supplies a pulsedoutput voltage and a constantV average power to a sputter-ion pumpoperating at high pressure.

Referring now to FIGS. 1-3 there are shown diagrams which illustrate thedramatic effect that cathode surface conditions have on the hydrogenpumping speed of a sputter-ion pump. The diagram of PIG. l represents aparticular sputter-ion pumps hydrogen pumping speed in liters per secondand length of pumping time in hours plotted on the vertical and thehorizontal axes, respectively. The pumping measurements were taken for a-sputter-ion pump whose cathodes had been exposed lto air and whilemaintaining a hydrogen environment pressure of approximately l 105 mm.of Hg within the sputter-ion pump envelope. The dotted curve llshows aconstant pumping speed of about 2 liters per second for the first 50hours of operation. Thedotted curve then indicates a gradual speedincrease between 50 and hours of operation time and then a leveling olfto a constant pumping Ispeed at approximately 4 liters per second. Thismodest speed increase in probably due to increased cathode permeabilitywhich results from rifts and cracks created in the cathode by theextended period of hydrogen ion bombardment.

The hiatus 12 between the dotted curve l1 `and the ltering to provide a.clean cathode surface.

` age.

`merit produced Vby the introduction ot argon into the pump envelope.The solid curve i3 shows an extremely high hydrogen pumping speed ofover 14 liters per second when the pump was returned to a hydrogenenvironment and measurements taken underv the same conditionsY as thosefor dotted curve gli. This extremely high pumping-speed then declinedgraduallyy over a period of approximately 30 hours to reach thefs'teadystate pumping speedof approximately ll liters per second illustrated. inthe last portion of the solid curve i3.

Thus, the curves of FIGQI illustrate the greatlyincreased hydrogen,pumpingV speed -which is obtainable after a sputter-ion pump hasoperated in an argon environment. Thisincreased speed results from themore vrapid rate Yof hydrogen molecule diffusion into the pump cathodewhich is possibley afterV the ion pumps reactive cathode surfaces Vhave`been sputtered clean of impurities by the 'bombarding-heavy argon ions.The intrinsic pumping speeds are `enhanced even more than the measuredones shown in FIG. l since the conductance effects are increased at thehigher pumping seeds. The ex- `tremely yhigh hydrogen pumping speedimmediately following the argon treatment V resultsV from the pumpingcontribution made by the freshly sputtered cathode material which hascollected on the sputter-ion pumps'anode and envelope surfaces.saturated with hydrogen, pumping occursl only into the cleaned cathodesurfaces 'and accordingly pumping speed t falls to the lower steadyvstate value illustrated by the solid curve 13. Also, the higlrvalue ofVsteadystate pumping speed illustrated by the solid Vcurve t3 of FIG.

l1 shows that the limited cathode sputtering produced by the lighterhydrogen ions is suiiicient to maintain a clean cathode in spite of thelimited amount of impuri- .ties in the hydrogen environment. Thisextremely high gases which are chemicallyinert with respect tothercathode material and which are of a greater molecular massthanrhydrogen will also produce suiiicient cathode sput- Furthermore,combinations of cleaning agent gases can be used.

, FIG. 2 shows a curve 14 which illustrates the residual pumpingcharacteristic whichV a] particular sputter ion pump exhibits lforhydrogen. Residual hydrogen pumping speed in liters per second isplotted on the vertical axis and time in seconds after turning ot thepump anode hydrogen leak into the pump envelope was held constant.

When this material has become'V curves of a particular sputter-ion pumpbefore and after argon treatment. Hydrogen Vpumping speed in liters persecond is plotted on the verticalA axisrvs. relative time` on thehorizontal axis. Dotted curve 1S represents the pumping speed 'of asputter-ion Vvacuum pump before treatment. As shown, the pump exhibitedan initial steady state pumping speedY of about two liters per secondunder normal operating voltage. At point lo the pumps oper ating Voltagewas reduced by 59% which resulted ina corresponding reduction in pumpingspeed to about one liter per second. Axt point i7 the full normaloperating voltage was'again applied andthe steady state pumping speedreturned to its-former value of two liters per second.

The solid curve 18 represents the hydrogen pumping speed of the samesputter-ion vacuum pump after having been treated by several hours ofargon'pumping. As shown, anrinitial steady state pumping speed of about14 liters per second was obtained with normal yoperating voltage. Y Atpoint 19 the pumps operatingy voltage was reduced by 50% and acorresponding reduction in steady Vstate pumping speed to about 13liters per second occurred. At point 21. normal pump voltage was againapplied and the steady `state pumping speed returned to its former valueof about i4 liters per second.l

' liter per second .for both the treated Vand the untreated sputter-ionpump. This occurredin spite of the fact that Y pumping speed for thetreated pump was-some 7 times-the pumping speed for the unthe steadystate hydrogen treated pump.' It thus appears that` the amount ofpumping speed produced by the pumps normal ion pumping lmechanism isrthesame for the treated or untreated pump.

This is representedrin the case of the treated sputter-ion Y pump by the`area between the solid curve i3 and a horizontal line 22 drawn atapproximately l2 liters per second. rThe remainder of the total pumpingspeed achieved by they treated sputter-ion pump is represented by all ofthe area below the horizontal line 22 and amounts to a constant pumping`speed of about -12 liters per second.

r:This pumping speed is apparently independent of the pump operatingvoltage andV Vis attributed to molecular pumping wherein tree moleculesof'hydrogen are freely adsorbed onto the Vclean cathode surfaces wherethey are dissociated and diuse into the cathode. In the case of theuntreated pump the presencefof gases exhibiting a reaction With thecathode materials apparently creates a` limits this type of molecular vlayer on the cathodes which pumping. g

FIGS. 4'and 5 illustrate the second important factor in the pumping ofhydrogen, i`.e.,`V the solubility. of the f t I bulk cathode materialVfor hydrogen. FIG. 4 shows the dependence of this solubility upon thetemperature of the cathode material with curve 23 plotting hydrogen Ypumping Ispeed liters per second on the vertical axis f and the cathodetemperature in degrees centigrade on they horizontal axis. Themeasurements were taken at a con Initial conditions for the curve wererapumping speed of l5 liters per second and a pressure of 4 l05 mm; Vof

Va factor of e was l l03 seconds. V

vThus, a sputter .ion pump willcontinue to pump hydrogen at asignificant rate for some time atter'its gas discharge has beenextinguished by` removing anode volt- The-gradual deterioration of theresidual hydro- Y. gen pumping speed is attributed to'- a contaminationofi the pum-p cathode surfaces'by impurities in the l1) drog Supply@ 'Yt y j .Y l

The diagramV of Pi-3 4'shows hydrogenk pumping peY ,Y falls off rapidlyto, ecorneginsigniiicant at 400 v centirs-tant'pressure of 5x10-5 mm.AHg andr as shownk the k pump exhibits a substantially constant pumpingspeed of about Zliters per second forcathode temperatures up to v about280 centigradc. Y Aft that point* the pumping speed grade. Thisdecreasey in hydrogenV pumping speed istatytributed to the decreasedsolubility 4,of the cathode material for hydrogen.

l FIG. 5( is a diagram of various power operating curves Vfor a typicalsputter-ion pumpjnominal pump-ing speed 5 Vl./s.) with pump input powerin Vwatts plotted on the vertical scale'and pumpyoperating pressureplotted on the f horizontal scale. The ldot-dash curve 26.1 representsthe Y', power input of an unlimited power supply-. As shown the "Y,power inputgrises linearly with increasing pressure. Solid Y curve2/5`shows the powerrcurve fora typically used ksputter-ionpower-supplyphaving some type of current'limiting device to limit powerinput to the pump in the pressure range of its starting condition(reduction of pump pressure from about -2 mm. Hg to below 10mi mm. Hg).Such current limited power supplies are used to prevent high powerthermal damage to the pump in this starting pressure region.

Some diiculty is experienced in the pumping of hydrogen with powersupplies of the type represented by the solid curve 25. The ditiicultiesresult from the large power hill 27 which must be overcome in order tostart the pump. The high power `that must be dissipated dur- -ingoperation in the upper portion of the power hill 27 produces highcathode temperatures. These temperatures frequently rise above thecritical value causing a reduction in the solubil-ity of the cathodematerial for hydrogen and a corresponding increase in re-evolution ofhydrogen to thereby greatly reduce the pumps hydrogen pumping speed.This greatly prolongs the time necessary to start the pump. For example,in nonnal operation using a power supply with power characteristic shownby the soiid `curve the time for starting a typical sputterion pump in ahydrogen atmosphere can be over one hour while the starting time for thesame pump wit-h other gases would be a few minutes.

Dotted curve 26 represents the power curve for a power supply operatedin accordance with the present invention. The input power tothe pumprises linearly with increasing pressure up to about 10-4 mm. of Hg andthen levels off to supply a constant ave-rage p-ower with respect topressure in the pressure region of the pumps starting condition.

By eliminating the power hill in the pumps starting pressure range, theproblem of heated cathodes and reduced solubility is eliminated and pumpstarting time is greatly reduced (typically to a few minutes).

Referring now to FIG. 6 there is shown a power supply circuit forproviding a sputter-ion pump with a substantially constant average powerinput with respect to pressure in the pressure region between 10-2 mm.of Hg and 10-4 mm. of Hg. The positive lead 31 of a conventional powersupply 32 ris connected to the anode 53 of a gas diode 34 whose cathodeelectrode 35 is connected through a series resistor 36 to t-he anode 37of ya thyratron 38. The cathode electrode 39 of thyratron 38 isconnected to the anode 41 of a sputter-ion pump 42 whose cathodeelectrodes 43 are connected to the pump envelope ed. A parallel resistor45 is connected directly between the anode 37 and the cathode 39 of thethyratron 38 while a capacitor 4o is connected between ground and thejunction between series resistor 36 .and theJ t-hyratron anode 37. Theprimary winding 47 of a transformer 48 is connected to the A.C. powersupp-1y 32 through a voltage regulator 49. The secondary winding 51 ofthe transformer 4S is connected in series with a resistor 52 and DC.power supply 53 between the grid 54 and the cathode 39 of thyratron 38.

In operation of the circuit of FIG. 6 the A.C. voltage applied to theanode 33 of the diode 34 and the AC. voltage applied to the grid 54 are180 out of phase. The diode 34 conducts only on positive cycles toprovide a pulsed D.C. current which charges the capacitor 4e toestablish a vol-tage at the anode 37 of the thyratron 3 When thecapacitor 46 has been charged to a sufficiently high voltage thethy-ratron 38 will ire as the A.C. voltage applied to the thyratron grid54 is going positive. Now, if the suputter-ion pump 44tis operating athigh pressure (between 10-4 mm. Hg and 102 mm. Hg, for example) theimpedance between the pump anode di and the pump cathodes 43 is verysmall. Therefore, the capacitor 46 will discharge through the conductingthyratron 3S and sputter-ion pump 42 to establish a gas dischargebetween the pump anode 41 and pump cathodes 43; As the capacitor 46discharges-the voltage at the thyratron anode 37 is reduced to cut ott"the thyratron 33 and de-energize the pump anode 4l..

Thus the circuit of FIG. 6 will provide intermittent pump operation athigh .pressures thereby reducing the pump input power. At the same timethe hydrogen pumping speed of the sputter-ion pump 42 remains high asthe intermittent operation of the gas discharge provides sulhcient ionsputtering to maintain clean pump cathodes while hydrogen pumpingcontinues even during the gas discharge oli periods due to the residualpumping effect shown in FIG. 2. The particular voltage duty cycleapplied to the pump anode 41 can be established by adjusting the timeconstant of the capacitor 46. The pump input power reduction isincreased by extending the oit portion of the pumps duty cycle andsputterion pumps have maintained high hydrogen pumping speeds whilleoperating, for example, on a duty cycle hav- -ing otlt portions whichwere about seven times as long as the on portions.

However, when the sputter-ion pump 42 is operating at low pressure(below 10-4 mm. Hg, for example) the impedance of the gas discharge ishigh so that the capacitor can not fully discharge in the periodsbetween charging pulses supplied by the diode 34. Therefore thethyratron 38 is not cut olf and a rippling D.C. voltage is applied tothe pump anode 41 through the continuously conducting thyratron 38. And,at extremely low pump pressures the pump impedance can become so largethat the current drawn by the pump 42 is insuicient to maintain thedischarge in the thyraton 38. The D.C. voltage wil then be applied topump anode 41 through the high resistance (K, for example) parallelresistor 45.

The pulsing circuit of FIG. 6 is a preferred embodiment because of thelarge pump input power reduction it provides. However, other types ofpower reducing circuits can also be used. For example only, the seriesresistor 36 can be replaced with an inductance which would also reducepump input power in addition to reducing power lost in the power supplycircuit.

Many other changes could be made in the above construction and manyapparently widely dilerent embodiments of this invention could be madewithout departing from the sco-pe thereof. It is therefore intended thatall matter contained in the above description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:

1. The method of treating a sputter-ion pump having anode and cathodeelectrodes adapted to maintain a gas discharge within a vacuum envelope,said method comprising the steps of initiating a gas discharge betweensaid anode and cathode electrodes, introducing into said envelope a gaswhich is chemically inert with respect to said cathode electrode andwhose molecules are of a greater mass than those of hydrogen, andbombarding said cathode electrode with ions of said gas so as to causesputering of said cathode electrode thereby providing a pump exhibitinga greatly increased pumping speed for hydrogen and its isotopes.

2. The method according to claim 1 wherein said gas introduced into saidenvelope is argon.

3. The method of treatin-g a sputter-ion pump having anode and cathodeelectrodes adapated to maintain a gas discharge within a vaccumenvelope, said method comprising the steps of introducing into saidenvelope a gas which is chemically inert with respect to said cathodeelectrode and whose molecules are of a greater mass than those ofhydrogen, initiating a Ygas discharge between said anode and cathodeelectrodes, and bomb-arding said cathode electrode with ions of said gasso as to cause sputtering of said cathode electrode thereby providing apump exhibiting a greatly increased pumping speed for hydrogen and itsisotopes.

4. The method according to claim 3 wherein said gas Q7 a gas dischargewithin a vacuum envelope, said method comprising the steps of evacuatingsaid vacuum envelope to a pressure below 10"2 mm. Hg, applying a pulsedVoltage to saidanode electrode of said sputter-ion pump,

' and said pulsed voltage having a duty cycle wherein the ott" portionisv greater than the on portion. f6. The method of operating asputter-ion pumpihaving anode and cathode electrodes adapted to maintainn a gas discharge within a Vacuum envelope, said method comprising thesteps of initiating ,a gas discharge between said anode and cathodeelectrodes, introducing Vinto said envelope a gas which is chemicallyinert with respect'to said cathode electrode and Whose molecules are ofa greater mass than those of hydrogen, and bombarding said cathodeelectrode with ions of 'said' gas, introducing hydrogen into said vacuumenvelope, and

applying a pulsed voltage to said anode electrode of said sputter-ionpump.

y7. The method according to claim 6 wherein said inert gas introducedinto said envelope is argon.

S.Themethod of operating a sputter-ion pump having anode and cathode`electrodes adapted to maintain a gas-discharge within a vacuumenvelope, said method comprising the steps of introducing into saidenvelope a gas which is chemically inert with respect to said cathodeelectrode and. whose molecules are Vof a greater mass than those'ofhydrogen, initiating a gas discharge between said anode and cathodeelectrodes, and vbombarding said .cathode electrode with ions of: saidgas so as to cause sputtering of saidrcathode electrode, introducinghydrogen into said vacunm'envelope, and applying a puised voltage toysaid anode electrodeof Vsaid spuuter-ion pump. 9. The method accordingto claim 8 wherein said inert gas introduced into said envelope isargon.

than those of hydrogen, and bombarding said cathodeV electrode with ionsof said gas to causersputtering of said cathode electrode therebyproviding a cleaned cathode surface which exhibits a greatly increasedpumping speed for hydrogen and its isotopes.

l1. The method according to claim l0 wherein said gas introduced isargon.

` 12. An apparatus for'pumping hydrogen and its isotopes comprising Vananode electrode, a cathode electrode, said anode and cathode .electrodesadapted to maintain a gas discharge upon energization of said anodeelectrode, power supply means for providing voltage to said anode elec-Vi trode meansfor adapting said power supply means to provide said anodeelectrode with a pulsed voltageV having aduty cycle wherein theY offportion is greater than the on portion. i

i3, The method of operating a sputter-ion `pump hav- Y ing anode andVcathode electrodes adapted to maintain a gas discharge within a vacuumenvelope, Ysaid methodV comprising the step of applying Vto said anodeVofvsaid sputter-ion pump a pulsed voltage having Va vduty cycleV whereinthe ot portion is greater than the on portion.

References Cited in the le of this patent l UNTED STATES PATENTS2,993,638' Y Hallein Juif/72s, i961

1. THE METHOD OF TREATING A SPUTTER-ION PUMP HAVING ANODE AND CATHODEELECTRODES ADAPTED TO MAINTAIN A GAS DISCHARGE WITHIN A VACUUM ENVELOPE,SAID METHOD COMPRISING THE STEPS OF INITIATING A GAS DISCHARGE BETWEENSAID ANODE AND CATHODE ELECTRODES, INTRODUCING INTO SAID ENVELOPE A GASWHICH IS CHEMICALLY INERT WITH RESPECT TO SAID CATHODE ELECTRODE ANDWHOSE MOLECULES ARE OF A